I'll always prefer the Joan Jett version, but here's plenty of belly shots from Britney's cover of I Love Rock and Roll.
https://www.youtube.com/watch?v=ITuOddPeYoc
What year was this? Before her piercing clearly.
The answer to your question:
"I Love Rock 'n' Roll" is the fourth European single released by pop singer Britney Spears from the album Britney on 27 May 2002. The song was used in her 2002 movie Crossroads. Spears's character, Lucy, performs it in a karaoke bar."
And my reaction to us still putting Britney Spears on a pedestal as the sole credit for the navel baring fashion trends in the late 90s/early 2000s, when at the same time, you had Shania Twain, Christina Aguilera, P!nk, Mariah Carey, Janet Jackson, Jennifer Lopez, The Spice Girls, and TLC, among others.
